There could be several advantages to an undersea vacuum-tube. It could be almost neutrally buoyant, and thus lightly tethered to the bottom, avoiding most of the shock and strain from earthquakes. Being under water, there should be less hydrogen seeping in through the stainless steel. Having waves above the length of the track might allow electric field generators and maintenance pumps to be powered by renewable wave energy. Being underwater, tsunamis could roll right over it with no effect. The tube would become somewhat less accessible to people bent on destruction - they’d have to own a boat and scuba gear at a minimum; no plinking the pipe with 50-cal sniper rifles from a safe distance. Maintenance might be easier, as the whole tube might be floated up to the surface where an enclosing maintenance ship could proceed along its length, with access to all sides. Still crazypants, but engagingly so.
